The present invention relates to an optical single-mode fiber having low dispersion for the wavelength division multiplex operation (WDM) of optical transmission paths, which is made of a central fiber core, at least two inner fiber cladding layers, and of an outer fiber cladding layer (triple-clad fiber), the refractive index profile n(r) of the fiber not being constant as a function of the fiber radius r.
To be able to transmit ever greater data rates over single-mode fibers, the wavelength division multiplex method (WDM) is increasingly gaining in importance. In WDM operation of an optical transmission path, up to 80 to 100 channels having a spectral bandwidth of xcex94xcex are transmitted over one fiber. The number of channels that can be transmitted over one fiber of a given length is essentially limited by the fiber attenuation and dispersion at the wavelengths being used. Also, the channel spacing needed to ensure transmission quality means that the fibers must have a large enough spectral width for the transmission.
The fiberglass cables installed in the optical networks of telecommunications companies contain all-silica optical fibers, which are made of a fiber core and a fiber cladding. The minimum attenuation of all-silica fibers is within the third optical window, thus within the spectral region of around 1550 nm. In this wavelength range, powerful optical amplifiers are also available, e.g., erbium-doped fiber amplifiers (EDFA), which are used within the optical network to regenerate the transmission signals following a specific path section. For these reasons, the WDM system currently used is conceived for the third optical window.
In the case of pre-installed glass fibers, one can encounter the problem of dispersion. For normal standard fibers, the zero dispersion wavelength xcex0, at which no dispersion or only very slight dispersion of optical signals occurs, is xcex0≈1310 nm. This means that a signal transmitted with a wavelength of about xcex0 is not or only slightly distorted, in particular, the pulse width is retained. However, the attenuation in this range is greater than in the third optical window. The chromatic dispersion D(xcex) in the case of standard fibers is substantially wavelength-dependent and, for xcex=1550 nm, amounts to about 16 to 17.5 ps/(km*nm). If an optical signal having wavelengths of about 1550 nm is transmitted, the pulse width is enlarged due to dispersion. This effect is an obstacle to a high transmission capacity; a chromatic dispersion of 16 to 17.5 ps/(km*nm) is much too high for ultra-high bit rate systems.
To be able to use laid standard fibers in the third optical window, it is necessary to compensate for the dispersion, which entails considerable outlay. In this regard, one knows of dispersion-compensating fibers, for example, from U.S. Pat. No. 5,568,583, which, at 1550 nm, exhibit a very high negative dispersion of D≈xe2x88x92100 ps/(km nm). These dispersion properties are achieved by raising the refractive index of the fiber core and by lowering the refractive index of a first cladding layer in comparison to the refractive index of the outer fiber cladding, made of silica. For the application, the dispersion-compensating fiber is spliced onto a standard fiber, so that the signal that is separated by positive dispersion when propagating through the compensation fiber is compressed again by the negative dispersion. A dispersion that is high in terms of absolute value is necessary to keep the length of the compensation fibers to a minimum.
It is also known to use special dispersion-shifted DS fibers, which have a zero dispersion wavelength of about 1550 nm, for the third optical window. A DS fiber of this kind is known, for example, from U.S. Pat. No. 5,675,688. In principle, comparably to the dispersion-compensating fibers, the zero wavelengths are shifted through the use of a specific refractive index profile.
However, these DS fibers have decisive disadvantages when used in WDM operation. The dispersion curve D(xcex) does, in fact, intersect the wavelength axis at about xcex0=1550 nm, however, in comparison to the dispersion curve of standard fibers, it is merely shifted toward higher wavelength D values. Thus, near 1550 nm, it has a steep rise angle, i.e., a steep slope angle S(xcex0), which lies at about 0.09 ps/km*nm2. This applies comparably to 1300 nm standard fibers, as well. This means, that for xcex values, which differ from xcex0, one has to expect significant dispersion values, which rise virtually linearly with the spacing from xcex0. This is, of course, a serious disadvantage, which limits the usable WDM spectrum and, therefore, must be overcome. The second disadvantage of the DS fibers is the relatively small effective surface Aeff of the fibers, i.e., the small mode field diameter MFD (Petermann II) and MFDeff. They increase the nonlinear refractive index (Kerr coefficient) of the fibers and, thus, nonlinear effects (Brillouin und Raman scattering), which degrade the transmission quality.
Furthermore, to overcome the dispersion problem in the third optical window, optical monomode fibers have been developed as a replacement for standard fibers. In the relevant spectral region, the monomode fibers exhibit low chromatic dispersion, as well as low loss. From the company prospectus xe2x80x9cTrueWave(trademark) Single Mode Fiberxe2x80x9d of ATandT Network Systems, a fiber is known, which, for wavelengths of about 1540 to 1560 nm, exhibits a chromatic dispersion D of 0.8xe2x89xa6Dxe2x89xa64.6 ps/(km*nm), given a mode field radius of 4.2 xcexcm. Qualitatively, the refractive index profile n(r) shows a triangular core profile, the triangle resting on a broad platform, whose height makes up about one tenth of the height of the triangle. With respect to the silica glass value of n=1.4573 (outer cladding area), only positive n(r) values occur, if one assumes n=1.4573 as the zero level. One forgoes lowering the refractive index level, e.g., through incorporation of fluorine.
A dispersion-shifted fiber is also known from the EP Patent 0851 251 245 A2. For wavelengths of around 1550 nm, it exhibits a dispersion of 1.0 to 4.5 ps/nm/km, a dispersion curve gradient of less than 0.13 ps/nm2/km, and an effective surface of at least 70 xcexcm2. The core of the fiber is subdivided into four layers, each having a different refractive index level. Contiguous to this fiber core is the outer fiber cladding layer. Thus, it is a quadruple-clad fiber. Another quadruple-clad fiber having at least four levels with a flat dispersion curve (0.03 ps/nm2/km) is known from WO 97/33188. To achieve the desired optical properties, the inner core level must be substantially increased in comparison to the reference refractive index of the outer clad level. In this context, close radius tolerances must be observed, in order to accommodate four layers. It is difficult to produce a refractive index profile with close radius tolerances on a regular basis, in the case where the profile varies considerably within the range of only a few micrometers. For the manufacturing, a plasma CVD process is suited. It enables fine layer structures of this kind to be precisely deposited. This process requires substantial outlay.
The usable spectrum in the third optical window is limited by the spectral operating range of the optical amplifiers (EDFA) used, which is between about 1510 and 1570 nm. However, since glass fibers, once installed, must be available for many years, one should anticipate future technical development and set the usable operating range of the fibers to be much higher, for instance between 1400 and 1700 nm.
From the EP 0 732 119 A1, a fiber is known, whose fiber core is partitioned into three or four layers, each having a different refractive index level, the maximum value of the refractive index deviation occurring within each layer being given by a reference value, and the dispersion within the wavelength range of 1400 to 1700 nm assuming values between xe2x88x927 ps/(nmxc2x7km) and +5 ps/(nmxc2x7km).
The present invention provides a single-mode WDM fiber having a plurality of layers, each with a different refractive index level, for use in an ultra-high bit rate transmission system, which, given a fiber profile that is technologically simple and cost-effective to produce, has a usable operating range of preferably between 1400 and 1700 nm, a normally large, effective surface or mode-field radius, and a dispersion characteristic D(xcex), which, in the spectral region under consideration, is as flat as possible and assumes D(xcex) values having a maximum amount of 3.7 ps/(nm*km).
Another embodiment of the present invention provides a single-mode optical fiber having low dispersion for the wavelength division multiplex operation (WDM) of an optical transmission path, made up of a central fiber core having a radius r1, two inner fiber cladding layers having an outer radius r2 and an outer radius a, respectively, where a greater than r2, and an outer fiber cladding layer, the refractive index profile n(r) of the fiber not being constant as a function of the fiber radius r, and the outer fiber cladding layer, i.e., for the region r greater than a, having a relative refractive index profile xcex94(r), where             Δ      ⁢              xe2x80x83            ⁢              (        r        )              =                  1        2            ⁢              (                                                            n                ⁡                                  (                  r                  )                                            2                                      n              c              2                                -          1                )              ,
for which it holds that xcex94(r)≈0, nc being a constant reference refractive index; and has for the radii r1, r2 and a, as well as for the relative refractive index profile xcex94(r), where       Δ    ⁢          xe2x80x83        ⁢          (      r      )        =            1      2        ⁢          (                                                  n              ⁡                              (                r                )                                      2                                n            c            2                          -        1            )      
of the fiber, the following holds:
a) 9.0 xcexcmxe2x89xa6axe2x89xa615 xcexcm, 0.15xe2x89xa6r1/axe2x89xa60.4, and 0.65xe2x89xa6r2/axe2x89xa60.85,
b) in the fiber core, i.e., for rxe2x89xa6r1, it holds that xcex940 xe2x89xa7xcex94(r)xe2x89xa70, where 3.5xc2x710xe2x88x923xe2x89xa6xcex940xe2x89xa66.0xc2x710xe2x88x923;
c) in the first inner fiber cladding layer, i.e., for r1 less than rxe2x89xa6r2, it holds that 0xe2x89xa7xcex94(r)xe2x89xa7xcex941, where 2.0xc2x710xe2x88x923xe2x89xa6xcex941xe2x89xa60.6xc2x710xe2x88x923;
d) in the second inner fiber cladding layer, i.e., for r2 less than rxe2x89xa6a, it holds that xcex942xe2x89xa7xcex94(r)xe2x89xa70, where 1.0xc2x710xe2x88x923xe2x89xa6xcex942xe2x89xa62, 0xc2x710xe2x88x923,
so that, within the wavelength range of between 1400 and 1700 nm, the fibers have a dispersion value of between xe2x88x921.6 and +3,7 ps/(nmxc2x7km).
In this context, nc is a constant reference refractive index, namely the refractive index of the outer cladding, which, as a rule, is made silica glass, where nc=1.4573.
For small differences in refractive indices, as exist here, the relative refractive index defined by             Δ      ⁢              xe2x80x83            ⁢              (        r        )              =                  1        2            ⁢              (                                                            n                ⁡                                  (                  r                  )                                            2                                      n              c              2                                -          1                )              ,
indicates approximately the absolute change in refractive index n(r)xe2x88x92nc, in terms of the cladding refractive index, since       Δ    ⁢          xe2x80x83        ⁢          (      r      )        ≈                              n          ⁢                      (            r            )                          -                  n          c                            n        c              .  
The first inner cladding layer is directly contiguous to the fiber core and is surrounded by the second inner cladding layer. The sequence of layers terminates with the outer fiber cladding layer having reference refractive index nc. Thus, the fiber in accordance with the present invention can be a triple-clad fiber.
The fiber core has a xcex1 profile (xcex94(r)=xcex940(1xe2x88x92rxcex1) where xcex1=1 . . . 6) or a trapezoidal profile, or has a constant refractive index (rectangular profile). The refractive indices in the remaining layers are preferably constant. A triple-clad fiber of this kind can be produced simply and cost-effectively, using conventional manufacturing methods as well.
In a further embodiment of the present invention, radius r1 is preferably between 2.5 xcexcm and 5.5 xcexcm, especially preferred is 3.5 xcexcmxe2x89xa6r1xe2x89xa64.5 xcexcm. For radius r2, values of between 8 and 12 xcexcm should be selected, preferably 9 xcexcmxe2x89xa6r2xe2x89xa611 xcexcm. For radius a, it holds preferably that 9 xcexcmxe2x89xa6axe2x89xa615 xcexcm.
In a further embodiment of the present invention, it holds that: xe2x88x921.2xc2x710xe2x88x923xe2x89xa6xcex941xe2x89xa6xe2x88x920.6xc2x710xe2x88x923.
A core profile form that can be easy to implement, i.e., for r less than r1, is a rectangular profile. In this context, the absolute and relative core refractive index for r less than r1 is more or less constant, and, in the range of r≈r1, it decreases to the value of the first inner cladding layer. Preferably, the three cladding layers likewise have a constant refractive index, which varies within the above indicated ranges.
In a further embodiment of the present invention, the following parameters can be selected for the fibers having a rectangular profile of the fiber core:
a) 3.7xc2x710xe2x88x923xe2x89xa6xcex940xe2x89xa64.6xc2x710xe2x88x923, preferably xcex10≈4.16xc2x710xe2x88x923;
b) 1.8xc2x710xe2x88x923xe2x89xa6xcex941xe2x89xa61.4xc2x710xe2x88x923, preferably xcex941≈xe2x88x921.59xc2x710xe2x88x923;
c) 1.6xc2x710xe2x88x923xe2x89xa6xcex942xe2x89xa61.9xc2x710xe2x88x923, preferably xcex942≈1.75xc2x710xe2x88x923;
d) 9.4 xcexcmxe2x89xa6axe2x89xa611.4 xcexcm, preferably a≈10.4 xcexcm, 0.15xe2x89xa6r1/axe2x89xa60.4, preferably r1/a≈0.3, and 0.65xe2x89xa6r2/axe2x89xa60.85, preferably r2/a≈0.8.
Another form of the core profile can be a triangular profile, xcex94(r) assuming the maximum relative and, thus, also the absolute refractive index xcex940 near the fiber midpoint and, up to r≈r1, decreasing linearly to the value of the first inner cladding layer. Contiguous thereto are cladding layers having a constant refractive index of xcex941, xcex942 and, respectively, xcex943=0.
In a further embodiment of the present invention, the following parameters can be selected for fibers whose core has a triangular profile:
a) 4.7xc2x710xe2x88x923xe2x89xa6xcex940xe2x89xa65.8xc2x710xe2x88x923, preferably xcex940≈5.31xc2x710xe2x88x923;
b) 1.0xc2x710xe2x88x923xe2x89xa6xcex941xe2x89xa6xe2x88x920.8xc2x710xe2x88x923, preferably xcex941xe2x88x920.9xc2x710xe2x88x923;
c) 1.1xc2x710xe2x88x923xe2x89xa6xcex942xe2x89xa61.4xc2x710xe2x88x923 preferably xcex942≈1.25xc2x710xe2x88x923;
d) 12.9 xcexcmxe2x89xa6axe2x89xa614.9 xcexcm, preferably a 13.9 xcexcm, 0.15xe2x89xa6r1/axe2x89xa60.4, preferably r1/a≈0.3, and 0.65xe2x89xa6r2/axe2x89xa60.85, preferably r2/a≈0.8.
In a further embodiment of the present invention, the core profile can be a parabola profile of the relative or of the absolute refractive index, the maximum relative and, thus, also absolute refractive index xcex940 being assumed in the vicinity the fiber midpoint, and xcex94(r) up to r≈r1 decreasing more or less parabolically to the value of the first inner cladding layer. Preferably contiguous thereto, in turn, are cladding layers having a constant refractive index xcex941, xcex942 or 0. Fewer mechanical tensions result when there is a continuous transition of refractive indices into one another. For that reason, under certain conditions, a parabola profile of the fiber core can be more stable than a rectangular profile.
In a further embodiment of the present invention, the following parameters can be selected for the fibers where the fiber core has a parabola profile:
a) 3.9xc2x710xe2x88x923xe2x89xa6xcex940xe2x89xa64.8xc2x710xe2x88x923, preferably xcex940≈4.34xc2x710xe2x88x923;
b) 1.1xc2x710xe2x88x923xe2x89xa6xcex941xe2x89xa6xe2x88x920.9xc2x710xe2x88x923, preferably xcex941≈xe2x88x921.03xc2x710xe2x88x923;
c) 1.3xc2x710xe2x88x923xe2x89xa6xcex942xe2x89xa61.7xc2x710xe2x88x923, preferably xcex942≈1.5xc2x710xe2x88x923;
d) 12.0 xcexcmxe2x89xa6axe2x89xa614.0 xcexcm, preferably a≈13.0 xcexcm, 0.15xe2x89xa6r1/axe2x89xa60.4, preferably r1/a≈0.3, and 0.65xe2x89xa6r2/axe2x89xa60.85, preferably r2/axe2x89xa60.8.
The profile specifications can be understood as theoretical setpoint entries. In practice, drastic jumps in the refractive index are not able, as a rule, to be precisely implemented; rather all corners of a theoretical profile are rounded off. The refractive index characteristics can be produced by depositing thin layers, so that, in practice, even a theoretically constant n(r) has a wave-shaped characteristic. Therefore, the above explanations refer to the target specifications. Moreover, at r=0, for example, a theoretically rectangular core profile often has a so-called middle dip, a decline in the refractive index and, therefore, merely a refractive index characteristic that can be approximated by a rectangle. The middle dip can be avoided through improved technology in the manufacturing process.
The described profiles are able to be produced using conventional modified chemical vapor deposition (MCVD) techniques.
The fiber in accordance with the present invention can have a chromatic dispersion D, which, for wavelengths from 1400 nm to 1700 nm lies within the range of between 1.6 to 3.7 ps/(nm km) and, thus, substantially below the value of standard all-silica fibers. Given a careful manufacturing of the fibers, the simple structures and relatively large field radii can ensure small polarization mode dispersion (PMD) values of less than 0.5 ps/kmxc2xd.
The fibers can be made for the most part of silica glass, which is doped with appropriate materials, preferably with germanium or fluorine, in order to raise or lower the refractive index in the core and in the cladding layers. Since the core doping required to reach the differences in refractive indices varies within the usual range, one should not expect the attenuation values of the WDM fibers to be higher than the current standard.
In addition, the core radii and, in particular, the effective radii weff of the fibers in accordance with the present invention are comparable to corresponding values of standard fibers. Since these quantities determine the polarization mode dispersion and the quantity of non-linear effects, both the PMD as well as the non-linear Kerr coefficient are comparable to those of standard fibers.
When fibers in accordance with the present invention are used, the transmission quality and transmission power of an optical transmission route can be enhanced as compared to standard optical fibers. Moreover, since a stabler signal form results from the lower dispersion, the glass fiber distance between the transmitter and receiver, respectively amplifier station, can be lengthened, which represents a cost savings. The small amount of pulse broadening makes it possible for substantially higher data rates to be transmitted, which is a prerequisite for ultra-high bit rate transmission systems.